1. Field of the Invention
The present invention relates to a visual search control of digital video files used in a video-on-demand system in which a desired one of the digital video files stored in a storage unit at a center is read out and transferred to a user terminal for the display in response to the user's request, which facilitates fast forward and fast reverse modes in addition to a normal playback mode and a switching of these operation modes in accordance with the user's request.
2. Description of the Background Art
Conventionally, the extended data such as the digital video data are managed in forms of sequential files on magnetic recording tapes operated by the VTR (Video Tape Recorder). In such a conventional digital video filing system using magnetic recording tapes, the fast forward and fast reverse modes have been realized by increasing the feeding speed of the tape from that of the normal playback mode while skipping as many frames as necessary.
In a case the digital video files are stored in a randomly accessible storage unit such as a magnetic disk, a read head is moved appropriately to skip as many frames as necessary.
An exemplary configuration of a conventional video-on-demand system using such digital video files is shown in FIG. 1, in which a plurality of user terminals 51 are connected to a center 53 through a network 52, where the center 53 comprises a storage unit 54 such as a magnetic disk for storing the digital video files, a storage unit interface 55 for controlling accesses to the storage unit 54, and a network Interface 56 for controlling connections of the center 53 with the network 52.
In this video-on-demand system of FIG. 1, the request 57 from each user terminal 51 is transferred to the network interface 56 of the center 53 through the network 52. In response to the received request, the storage unit interface 55 reads out the appropriate digital video data 58 from the storage unit 54 and transfer the read out digital video data 58 to the user terminal 51 which issued the request through the network 52 such that the desired digital video data 58 can be displayed on the user terminal 51. It is also possible for some user terminals 51 to be directly connected with the center 53, without using the network 52, if desired.
In this video-on-demand system of FIG. 1, the visual search control between the normal playback mode and the fast forward mode, for example, is carried out according to the timing chart of FIG. 2 as follows.
Namely, the normal playback mode is carried out by reading the video files in units of groups, one group per each cycle, where each group contains a predetermined number of frames to be read out together in one cycle. On the other hand, the fast forward and fast reverse modes are carried out by reading in units of frames, with a prescribed number of frames skipped between successive readings of the frames, in each cycle.
Thus, in an exemplary case depicted in FIG. 2, after the data transfer at the cycle-1, the normal playback mode is continued up to a middle of the cycle-2 at which a mode switching request 41 to switch from the normal playback mode to the fast forward mode is received from the user terminal. When this mode switching request 41 is received, it is necessary to skip to a first frame to be read out in the fast forward mode, so that there is a mode switching period of 0.06 sec. as explained below, between the receiving of the mode switching request 41 and the actual mode switching from the normal playback mode to the fast forward mode. During this mode switching period, the preceding mode of the normal playback mode is continued on the user terminal.
Then, the next mode switching request 42 to switch from the fast forward mode to the normal playback mode is received during the cycle-2. When this mode switching request 42 is received, it is necessary to read out one group of video files to be read out in the normal playback mode before the actual mode switching, so that there is a mode switching period of 0.2 sec. as explained below, between the receiving of the mode switching request 42 and the actual mode switching from the fast forward mode to the normal playback mode. During this mode switching period, the display on the user terminal is paused.
In this example, each video file containing video data as well as audio data is provided in a form of a bit sequence encoded at the bit rate of 6 Mbps. Thus, in a case of realizing 30 frames/sec. of the reading rate, each frame has 0.2 Mbits. For the sake of simplicity, it is assumed that one group is formed by 18 frames, so that each group has 3.6 Mbits. Now, when the access time of the storage unit 54 is 50 msec., and the transfer rate of the storage unit 54 is 24 Mbps, a time required for reading one group is given by: EQU 50 msec. +3.6 Mbits/24 Mbps=0.2 sec.
while a time required for reading one frame is given by: EQU 50 msec. +0.2 Mbits/24 Mbps=0.06 sec.
which are the mode switching periods used above.
In a case of the normal playback mode, it is only necessary to read one group per each cycle of 0.6 sec., whereas in a case of the fast forward and fast reverse modes, as many frames as necessary are skipped according to the desired searching rate. Here, however, since the reading of one frame requires 0.06 sec., a maximum number of frames that can be read out within one cycle is limited to 10.
Now, in this conventional visual search control, because the skipping of the frames is required in the fast forward and fast reverse modes, the overhead of the disk access such as the seek time and the latency time of a disk drive arises every time one frame is read out. Thus, when one frame in each two successive frames is skipped, it is only possible to read out 16 frames per sec., (1 sec./0.06 sec. 16.7 frames), which is much smaller than the reading rate of 30 frames per sec. realized in the normal playback mode. Consequently, the a number of frames that can be read out and displayed in the fast forward and fast reverse modes per unit time is decreased from that for the normal playback mode, such that the displayed video image in the fast forward and fast reverse mode is going to be an awkward and discontinuous poor quality image.